1. Field of the Invention
This invention relates to devices for generating near-field light. More particularly, the present invention relates to methods and structures for generating optimized near-field light for thermally assisted magnetic recording.
2. Description of the Related Art
Near-field light sources are useful for generating subwavelength, intense light for use in optical microscopes, optical measurement instruments, spectroscopic instruments, optical recording and optical reproduction equipment, lithography equipment, and for thermally assisted magnetic recording. In the latter application, heat is applied to a magnetic substrate via a very small, but intense light source to reduce the anisotropy of fine grain magnetic structures. These fine grain structures are capable of high recording densities, but have an anisotropy at room temperature that is too high for typical magnetic fields produced by conventional recording heads. Heating allows the media to be written with field strengths of conventional heads. Another potential advantage is that lower field strengths produced by heads having broader field dimensions may also be used, which relaxes the tight dimensional requirements of the magnetic source or write head. However, to be useful for high density recording, the light source utilized for heating must be on the order of 50 nm or less in diameter. This is far beyond the optical diffraction limit for conventional light sources such as solid state lasers.
One method that commonly used to produce such a near-field light source is the ridge aperture or “C” shaped aperture. The device consists of rectangular shaped aperture placed in an electrically conductive metal film, which is supported on a transparent substrate. The substrate is transparent to the wavelength of incident radiation, and covers the area of the aperture. Extending into the center portion of the aperture is an electrically conductive ridge, generally an extension of the surrounding metal film. Incident radiation, polarized in the direction parallel to the ridge produces a near-field light source which appears close to or at the end of the ridge, in the gap between the end of the ridge and the opposing boundary of the aperture. Numerous prior art references have described the formation of near field light sources using ridge apertures, many of which are summarized below. It is clear from these publications that intensity of the near field light source depends strongly on: (1) the particular dimensions of the aperture, which include the length and width of the outer boundary as well as the length and width of the ridge; (2) the thickness and composition of the metal film surrounding the aperture; and, (3) the frequency of light used to illuminate the aperture. For thermally assisted recording, heating of the recording media is of prime importance, so besides the variables cited above, the absorption characteristics of the media and location of the near-field light source above the media become additional parameters of importance. Most practitioners skilled in the art disclose information dealing with efficiency of the apertures, which is computed from the ratio of the light flux impinging on the entire aperture to the flux generated by the near-field source, with little regard to the impact of adsorption efficiency within the media being heated by the near-field source. Furthermore, since cost is always a concern in today's mass storage device technology, it would be useful to build a device to produce near-field light sources utilizing 780 nm wavelength light emitting diode light sources, which are low cost devices.
What is needed is a device for producing near field light sources for use in thermally assisted magnetic recording applications, optimized for low cost 780 nm wavelength illumination light sources, that is also optimized for maximum adsorption efficiency by the recording media.
Sendur et al., in an article entitled “Ridge waveguide as a near-field aperture for high density data storage”, Journal of Applied Physics, Volume 96, No. 5, September 2004, discloses the performance of the ridge waveguide as a near-field aperture in data storage systems. Finite element method (FEM) and finite-difference time-domain (FDTD) based software are used in the numerical simulations. To verify accuracy at optical frequencies, the FEM and FDTD are first compared to analytical results. The accuracy of these techniques for modeling ridge waveguides at optical frequencies is also evaluated by comparing results with plane wave illumination. The FEM, which is capable of modeling focused beams, is then used to simulate various geometries involving ridge waveguides. Near-field radiation from ridge waveguide transducer is expressed in terms of power density quantities. Previous studies in the literature consider the performance of the transducer in free space, rather than in the presence of a recording magnetic medium. The effect of the recording magnetic medium on the transmission efficiency and spot size is discussed using numerical simulations. The effect of various geometric parameters on the optical spot size and transmission efficiency is investigated and discussed. Based on our numerical simulations, a promising transducer design is suggested to obtain intense optical spots well below the diffraction limit. Numerical simulations suggest that a full width at half maximum spot diameter of 31 nm in the recording magnetic medium can be obtained. The maximum value of the absorbed optical power density in the recording medium is about 1.67×10−4 mW/nm3 for a 100 mW input power. This adsorbed power density is reported for a wavelength of 516 nm and silver conductive films. In-track and cross-track profiles for this design are compared with Gaussian distributions.
Batra et al., in an article entitled “Topology Visualization of the Optical Power Flow through a Novel C-Shaped Nano-Aperture”, published in IEEE Visualization 2004, discloses a study describing the application of flow topology visualization techniques to the investigation of optical transmission through a novel C-shaped metal nano-aperture. This feature based vector field visualization technique can be extended generally to the flourishing field of Finite-Difference-Time-Domain (FDTD) methods used in many nano-photonics problems. Because of the diverse applications of the FDTD method and the vast amount of information it produces, an effective high-level abstraction of the data set can not only provide critical insight into the physical phenomena, it will also help direct ongoing research efforts. Recently discovered C-shaped sub-wavelength (nano) metallic apertures, when irradiated at specific resonance frequencies, have extraordinary power transmission five to six orders of magnitude beyond what is observed for conventional round or square apertures. These apertures produce optical spot sizes as small as 25-50 nm using visible light in the near-field of the aperture with a brightness 10-100 times higher than that of the illuminating beam.
Shi et al., in an article titled “Design of a C Aperture to Achieve λ/10 Resolution and Resonant Transmission”, published in J. Opt. Soc. Am. B, vol. 21, No. 7, July 2004, discloses small subwavelength apertures providing high spatial resolution that is not limited by the diffraction limit. However, application of these apertures to practical problems has been hindered by the critical problem of extremely low power transmission efficiency. A well-designed C aperture can provide both a high spatial resolution of λ/10 and a high power throughput greater than 1. The authors present the underlying design ideas of the C aperture and report interesting general properties of optical transmissions through a single two-dimensional subwavelength aperture, based on numerical finite-difference time-domain simulations and fundamental observations. These results are expected to provide helpful information for both C aperture applications and general studies of subwavelength metallic structures.
Shi et al., in an article entitled “Ultrahigh Light Transmission Through a C-shaped Nanoaperture”, published in Optics Letters, Vol. 28, No. 15, Aug. 1, 2003, discloses that optical resolution beyond the diffraction limit can be achieved by use of a metallic nanoaperture in a near-field optical system. Conventional nanoapertures have very low power throughput. Using a numerical finite-difference time domain method, the authors discovered a unique C-shaped aperture that provides ˜3 orders of magnitude more power throughput than a conventional square aperture with a similar near-field spot size of ˜λ/10. Microwave experiments at 6 GHz quantitatively confirmed the simulated transmission enhancement. The high transmission of the C-aperture, or one of the related shapes, is linked to both a propagation mode in the aperture and local surface plasmons.
Shi et al., in an article entitled “Mechanisms for Enhancing Power Throughput from Planar Nano-Apertures for Near-Field Optical Data Storage”, published in J. Appl. Phys., Vol 41(2002), pp 1632-1635, discloses that near field optical data storage systems are important for ultrahigh density data storage. One type of near field optical data storage system uses a planar nanoaperture to define the data storage resolution. However, conventional nano-apertures have a tradeoff between resolution and power throughput. A “C” aperture design has been recently reported to overcome this problem. A study on the mechanisms for power throughput enhancement, based on a detailed study of nanoslits using numerical simulations is presented. Generating a propagation mode is essential for high power throughput, and by optimizing the design of the aperture thickness, resonant transmission can be achieved for even higher power throughput.
Matteo et al., in an article entitled “Fractal extensions of near-field aperture shapes for enhanced transmission and resolution”, published in Optics Express, Vol. 13, No. 2, Jan. 24, 2005, discloses families of fractals investigated as near-field aperture shapes. They are shown to have multiple transmission resonances associated with their multiple length scales. The higher iterations exhibit enhanced transmission, and spatial resolution exceeding the first order. Near-field enhancements of greater than 400 times the incident intensity and resolutions of better than λ/20 have been shown with apertures modeled after third iteration prefractals. Enhancements as large as 1011 have been shown, when compared with conventional square apertures that produce the same spot size. The effects of the complex permittivity values of the metal film are also addressed.
Matteo et al., in an article entitled “Spectral analysis of strongly enhanced visible light transmission through single C-shaped nanoapertures”, published in App. Phys. Letters, Vol. 35, No. 4, Jul. 26, 2004, discloses single C-shaped apertures designed, fabricated, and characterized in an Au film, resonant in the visible regime. The C-shaped apertures showed transmission enhancement of 13-22 times over a square aperture of the same area and suggest as high as 106 times enhancement over square apertures that are designed to produce the same near-field spot size. Spectra from individual apertures demonstrate the ability to tune this resonance over 70 nm by scaling the dimensions of the apertures. This shows the C aperture to be a versatile tool for gaining high-resolution, enhanced transmission through single subwavelength apertures at optical wavelength.
Chen et al., in an article entitled “Imaging of optical field confinement in ridge wave guides fabricated on very-small-aperture laser”, published in App. Phys. Letters, Vol. 83, No. 16, Oct. 20, 2003, discloses the optical field confinement in a ridge wave guide nano-structure (“C” aperture) designed for ultrahigh-density recording, observed using an apertureless near-field scanning optical microscope. The aperture was fabricated on a commercial edge-emitting semiconductor laser as the light source. High-contrast near-field images at both 1× and 2× lock-in detection frequencies were obtained. The emission patterns are in agreement with theoretical simulation of such structures. A 90 nm×70 nm full width half maximum spot size was measured and is comparable to the ridge width of the aperture.
Itagi et al., in an article entitled “Ridge waveguide as a near-field optical source”, published in App. Phys. Letters, Vol. 83, No. 22, Dec. 1, 2003, discloses the feasibility of using a ridge waveguide at optical frequencies as a near-field optical transducer, using the finite difference time domain method. The complete electromagnetic field picture of the ridge waveguide, in the absence and presence of the irradiated medium, is presented. A power efficiency of 7% and an optical spot with full width half maximum of 50 nm×80 nm is obtained in the medium. We show that impedance considerations play a major role in the transducer-medium optical coupling.
U.S. Pat. No. 5,696,372 discloses a near-field electromagnetic probe that converts an incident energy beam into an interrogating beam which exhibits, in the near-field vicinity of the probe, a transverse dimension that is small in relation to the wavelength of the incident energy beam. The probe comprises an energy source for providing the incident energy beam with a wavelength λ. An antenna is positioned in the path of the incident energy beam and comprises at least a first conductive region and a second conductive region, both of which have output ends that are electrically separated by a gap whose lateral dimension is substantially less than λ. The electromagnetic system which produces the incident energy should preferably have its numerical aperture matched to the far-field beam pattern of the antenna. Further, the incident beam should have a direction of polarization which matches the preferred polarization of the antenna. The near-field probe system of the invention can also sense fields in the near-field gap and reradiate these to a far-field optical detector. Thus the probe can serve to both illuminate a sample in the near-field gap, and to collect optical signals from an illuminated sample in the near-field gap.
U.S. Pat. No. 6,649,894 discloses an optical near-field probe of high resolution and high efficiency. A near-field light is generated using a tapered, plane scatterer formed on a substrate surface. The intensity of the near-field light is enhanced by making the area of the scatterer smaller than that of a light spot and by selecting the material, shape, and size of the scatterer so as to generate plasmon resonance. An optical near-field generator having a high light utilization efficiency can be obtained.
U.S. Pat. No. 6,714,370 discloses a recording head for use in conjunction with a magnetic storage medium, comprising a waveguide for providing a path for transmitting radiant energy, a near-field coupling structure positioned in the waveguide and including a plurality of arms, each having a planar section and a bent section, wherein the planar sections are substantially parallel to a surface of the magnetic storage medium, and the bent sections extend toward the magnetic storage medium and are separated to form a gap adjacent to an air bearing surface, and applies a magnetic write field to sections of the magnetic recording medium heated by the radiant energy. A disc drive including the recording head and a method of recording data using the recording head are also provided.
U.S. Pat. No. 6,768,556 discloses a near-field probe including a metallic scatterer fabricated on a substrate in a contour of a circular cone, a polygonal pyramid, a planar ellipse, or a triangle and a film of a metal, a dielectric, or a semiconductor formed in a periphery of the scatterer with film thickness equal to height of the scatterer.
U.S. Pat. No. 6,785,445 discloses a near-field light probe capable of emanating a near-field light having a sufficient intensity while allowing reduction of aperture size to improve resolution. The near-field light probe can be incorporated in a near-field optical microscope, a near-field light lithography apparatus, and a near-field light storage apparatus. A near-field light probe has a configuration in which a light-blocking film is formed with an aperture having slits surrounding the major opening. Light emitted from a light source is coupled into the probe from one side of the light-blocking film, the light being polarized in a predetermined direction with respect to the slits so that a near-field light emanates from the major opening.
U.S. Pat. No. 6,795,380 discloses a pair of members opposed to each other via a gap which are commonly used as an evanescent light probe and a writing magnetic head. When the spacing and width of the gap are smaller than the wavelength λ of injected light, highly intensive evanescent light is generated from the gap position of the opposite surface. Magnetic writing is carried out by applying a recording magnetic field from the pair of members to a medium heated by the evanescent light.
U.S. Pat. No. 6,839,191 discloses an optical near-field generating element provided with: a light shielding member, which is placed on an optical path of light emitted from a light source, for defining a micro opening having a diameter equal to or shorter than a wavelength of the light; and a dielectric film placed in close contact with the micro opening. Alternatively, an optical near-field generating element is provided with a light shielding member, which is placed on an optical path of lights emitted from a light source, for defining a micro opening having a diameter equal to or shorter than a wavelength of the light, the shielding member equipped with: a main portion for defining a basic shape of the micro opening; and a protrusion portion protruding from the main portion toward the center of the micro opening.
US Patent Application Publication 2003/0015651 discloses optical apparatuses using the near-field light where high spatial resolution and high sensitivity are made compatible. Highly intense near-field light is generated in a narrow area using localized plasmons that are produced in a metal pattern in the shape that bears anisotropy and is made to irradiate a measured subject. The direction of polarization of incident light is modulated and signal light is subjected to synchronous detection, so that background light is removed and high sensitivity is achieved.
US Patent Application Publication 2003/0184903 discloses a ridge wave guide having a recessed ridge forming an isolated tip at a terminal end of the wave guide's aperture which efficiently couples light into a spot adjacent to the tip in a medium below the terminal end of the wave guide that is significantly smaller than the light's wavelength. The wave guide is used to heat a recording medium via the small spot for heat assisted recording or to pattern substrates by photolithography with line width exposures that are significantly smaller than the wavelength of light. The body of the wave guide may also be recessed away from the medium in an area surrounding the wave guide's aperture to further confine the energy emerging from the wave guide to the small spot adjacent the tip in the medium.
US Patent Application Publication 2003/0223316 discloses a recording head for decreasing recording noise accompanying malformation of a recorded mark and the formation of a recorded mark capable of increasing reproduction resolution at the time of magnetic reproduction. The head has a light source and a scatterer for recording information on a recording medium by generating near-field light by application of light from the light source and forming a magnetic domain array on the recording medium, a perimeter of the scatterer defines a plurality of vertices and a distance between a first vertex and a last vertex is shorter than the width of the recording track on the recording medium. The recording head improves recording density and can be used to manufacture a highly reliable information recording and reproducing apparatus having a reduced cost per capacity.
US Patent Application Publication 2004/0062152 discloses a device for writing data to a recording medium and a method for fabricating the device. According to one embodiment, the device includes an electrical conductor having a cross-track portion, wherein the cross-track portion includes first and second opposing surfaces, and wherein the cross-track portion defines an aperture extending from the first surface to the second surface. The device also includes a dielectric portion disposed in the aperture such that the dielectric portion defines a ridge waveguide having a lowest-order mode cut-off frequency that is less than the frequency of incident optical energy used to heat the recording medium.
US Patent Application Publication 2005/0031278 discloses near-field sub-wavelength C-apertures that provide enhanced spatial resolution and power throughput by increasing the normalized resonant wavelength of the aperture. These improved apertures are characterized by the use of improved geometric proportions for C-apertures, filling the aperture with high-index material, designing aperture thickness to produce longitudinal transmission resonance, and/or tapering the aperture in the longitudinal direction to achieve impedance matching. Apertures according to the present invention may be used for many technological applications in various portions of the electromagnetic spectrum. Exemplary applications to high density optical data storage and optical particle trapping and manipulation are described.
US Patent Application Publications 2005/0030992 and 2005/0030993 disclose a near field optical apparatus comprising a conductive sheet or plane having an aperture therein with the conductive plane including at least one protrusion which extends into the aperture. The location, structure and configuration of the protrusion or protrusions can be controlled to provide desired near field localization of optical power output associated with the aperture. Preferably, the location, structure and configuration of the protrusion are tailored to maximize near field localization at generally the center of the aperture. The aperture preferably has a perimeter dimension which is substantially resonant with the output wavelength of the light source, or is otherwise able to support a standing wave of significant amplitude. The apparatus may be embodied in a vertical cavity surface emitting layer or VCSEL having enhanced nearfield brightness by providing a conductive layer on the laser emission facet, with, a protrusion of the conductive layer extending into an aperture in the emission facet. The aperture in the emission facet preferably has dimensions smaller than the guide mode of the laser, and the aperture preferably defines different regions of reflectivity under the emission facet. The depth of the aperture can be etched to provide a particular target loss, and results in higher optical power extraction from the emission facet.
International Publication WO 01/17079 discloses a near-field optical apparatus comprising a conductive sheet or plane having an aperture therein, with the conductive plane including at least one protrusion which extends into the aperture. The location, structure and configuration of the protrusion or protrusions can be controlled to provide desired near-field localization of optical power output associated with the aperture. Preferably, the location, structure and configuration of the protrusion are tailored to maximize near-field localization at generally the center of the aperture. The aperture preferably has a perimeter dimension which is substantially resonant with the output wavelength of the light source, or is otherwise able to support a standing wave of significant amplitude. The apparatus may be embodied in a vertical cavity surface emitting layer or VCSEL having enhanced near-field brightness by providing a conductive layer on the laser emission facet, with a protrusion of the conductive layer extending into an aperture in the emission facet. The aperture in the emission facet preferably has dimensions smaller than the guide mode of the laser, and the aperture preferably defines different regions of reflectivity under the emission facet. The depth of the aperture can be etched to provide a particular target loss, and results in higher optical power extraction from the emission facet.